Here's another Emergency gas planning example & exercise for the OP, for a dive to the Basic Open Water Limit of 18m/60 feet:
Emergency "Stressed" 28 litres/min SCR (Surface Consumption Rate, equivalent to 1 cuft/min in US/Imperial), with 18 meters (2.8 ATA) depth NDL, and with one minute stops every 3 meters/10 feet to surface [note: a one minute stop in this example is considered a 30 second hold at the present depth plus a 30 second ascent to the next depth]:
2.8 x 28 x 1 = 78.4
2.5 x 28 x 1 = 70
2.2 x 28 x 1 = 61.6
1.9 x 28 x 1 = 53.2
1.6 x 28 x 1 = 44.8
1.3 x 28 x 1 = 36.4
1.0 x 28 x 1 = 28
Sum Total: 372.4 litres gas needed to ascend to surface for an emergency contingency.
Divide the above total by the metric tank rating of the Scuba cylinder in use; for this example let's use the AL80 tank again which has a metric rating of 11 litres/bar. So 372/11 = approx 35 bar.
That's 35 bar to get you to the surface --to get yourself and sharing gas with an Out-of-Air Buddy you will need at least twice this amount: 35 x 2 = 70 bar. Therefore your Rock Bottom Reserve is an actual reading of 70 bar showing on your SPG --if there is no emergency air sharing contingency at that instant, just continue the dive normally but start a easy nominal ascent to the shallower depths between 5m and 9m. Finally, be at your 5m safety stop with your buddy with no less than 50 bar showing on the SPG.
Your usable gas for the dive is your starting pressure subtracted by the Rock Bottom Reserve --so a full AL80 tank of 200 bar minus 70 bar equals 130 bar usable. Let's use 20 bar of this 130 to get squared away in good trim & buoyancy on the descent to 18m which then leaves 110 bar usable. Now if your dive plan calls for returning to near the vicinity of your original point of entry (like a beach dive for instance), then turn the dive back around when you use half of 110 bar or 55 bar consumed. If you want an even more conservative value because of thick kelp for example, use rule-of-thirds - 1/3 of 110 is 36 bar in this instance.
So for a nominal dive with an SCR of 22 litres/min on a 11L/bar (AL80) tank, your resulting pressure SCR will be 2 bar/min [22 divided-by 11 equals 2bar/min]. Your depth in meters, which converts easily to ATA (simply divide-by-10 and add 1) becomes your multiplier depth factor for your 2bar/min pressure SCR.
18 meters depth is 2.8 ATA (divide 18 by 10 and add 1 equals 2.8); your 2bar/min SCR at depth -or Depth Consumption Rate (DCR)- now becomes 5.6 bar/min. [2.8 times 2bar/min equals 5.6 bar/min]. So 10 minutes at depth 18m on an AL80 (11L/bar) tank in nominal conditions, you would expect to consume 56bar of gas (10min times 5.6 bar/min equals 56bar) and your SPG reading to show a delta down of 56bar. . .
Okay, you splash in to start the dive with 200bar and a 70bar rock bottom. You use up 20bar on descent --initial exertion, inflating your wing etc. -so you now have 180bar with 70bar rock bottom at present depth 18m, a net usable of 110bar (180 minus 70 is 110). Finning out for 10 minutes, an easy swim looking at all the cool marine life around -- you already know by the end of this 10 minute interval that you've used up 56bar (see previous paragraph above), so you decide to turn the dive.
You should now know you have roughly 50bar of usable gas left before encroaching on your Rock Bottom Reserve, and realize that you must start a nominal ascent within the next 10 minute interval. Alright, so while turning back, you see a turtle and decide to chase it for a few minutes, huffing & puffing on your reg, until it dives down below your operational depth of 18m; because of this physical exertion you immediately check your SPG and it reads 70bar --bingo! Rock bottom has arrived so start ascending to the shallower depths . . .or if your Buddy just happens to blow his tank neck O-ring at that instant --you know you've got plenty of breathing gas margin for a controlled air-sharing Emergency ascent profile as described above. . .
Be aware that depending on environmental conditions and physical exertion (cold water, stiff current, long surface swim, heavy workload at depth etc), you may have to decide on-the-fly to reserve a greater Rock Bottom Reserve --perhaps as high or even over 100 bar (half tank) on the 11L/AL80 tank to be even more conservative.
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Notes for American Divers using US Imperial Units:
Stressed SCR of 28 litres/min is equivalent to 1 cuft/min;
Nominal SCR of 22 litres/min is approx 0.75 cuft/min.
2 bar/min is same as 29psi/min;
5.6 bar/min is 81psi/min
Easy imperial US/metric conversions for depth & pressure, that you can do in your head:
Depth in meters multiplied by 10/3 gives depth in feet;
Feet multiplied by 3/10 gives meters.
Example: 18m(10/3) = 60' ; 60'(3/10) = 18m
Pressure bar multiplied by 3/2, and multiplied again by 10 gives pressure psi;
Pressure psi multiplied by 2/3, and divided by 10 gives pressure bar.
Ex): 200bar(3/2)(10) = 3000psi ; 3000psi(2/3)/10 = 200bar.
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Your common counting numbers, or reference cardinal numbers, for depth in scuba are:
Imperial US (feet) by 10's:
Ex): 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110 etc
Metric system goes by 3's:
Ex): 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33 etc
It's easier to figure out ATA (Atmospheres Absolute depth multiplier factor) with meters:
Example: 24 meters is 3.4 ATA (divide 24 by 10 and add 1); the equivalent 80 feet in US/imperial is also 3.4 ATA, but not as easy to calculate (80 divided by 33 plus 1).
